Abstract We studied seven cases of myeloid leukemia at various differentiation stages to investigate the response of leukemic cells to phorbol 12- myristate 13-acetate (PMA) and various biological factors. gamma- Interferon (gamma-IFN)-treated cells expressed higher amounts of Fc receptors on leukemic cells in five out of seven cases. Expression of HLA-DR antigen of gamma-IFN-treated leukemic cells was significantly enhanced in three cases. PMA did not induce Fc receptors or HLA-DR antigen on these cells. Induction of Tac antigen, a putative interleukin 2 (IL 2) receptor, was observed in two cases after cultivation with PMA or with a novel lymphokine, adult T cell leukemia- derived factor (ADF). Cells from one of these patients expressed Tac antigen immediately after cell separation, and expression of Tac antigen was augmented by PMA and ADF. Interleukin 1 (IL 1) or IL 2 did not induce Tac antigen. Leukemic cells from this patient also proliferated vigorously in the presence of ADF but not PMA, IL 1, or IL 2.